1. Field of the Invention
The present invention concerns a process of fabricating a feeler member used in a micromechanical probe, in particular for an atomic force microscope or the like, which member can have one or more feeler tips. The invention also concerns a feeler member fabricated by this process.
2. Description of the Prior Art
Probes of this type for atomic force microscopes (AFM) are used in profilometry techniques, in particular, the tip or tips being made with a precision significantly better than one micron and being movable over submicron distances in three orthogonal directions to scan the surface of a sample to measure its topography at the atomic dimension level.
A number of processes are currently employed for fabricating feeler members of the type indicated above. For example, a pyramid-shape hole can be micro-machined, for example KOH etched, in a silicon substrate for use as a mold and the hole obtained in this way filled with silicon nitride or diamond, after which the silicon is removed to leave a tip used to explore the surface of a sample. A process of the above kind can be used to fabricate feeler members including a plurality of tips disposed side-by-side, for example.
Another process consists in forming silicon tips on a surface on which the silicon has been "undercut" isotropically or anisotropically.
Feeler members can also be made with carbon tips by depositing carbon using an electron beam or with tips obtained by etching using a beam of focused ions. Other techniques can be used entailing photo-structured glass or molding silicon nitride in holes obtained by RIE (reactive ion etching).
Of all the above techniques, those for obtaining diamond tips are generally preferred because of the very high Young's modulus of this material (1000 GPa compared to that of silicon which is only 170 GPa), which makes the tip very rigid, and because of the very high hardness of diamond, highly resistant to wear.
The fabrication of diamond tips uses the molding technique employing, for example, the fabrication process described in patent EP O 766 060 in which the diamond is grown onto a diamond nucleation layer previously deposited into a hole formed in a silicon substrate.
A preferred process of making such holes in the substrate is [111] anisotropic KOH etching, for example, and produces pyramid-shape holes, in principle with a square base with an angle of approximately 70.degree. between projections of the pyramid axis onto two opposite faces.
Probes with tips molded in holes having angles this large have a relatively low resolution, so fairly sudden discontinuities of the surface to be explored cannot be measured with the required precision. This is illustrated in FIG. 1 of the appended drawings showing that if, for example, the discontinuity is a step E (which can have a depth of 0.1 .mu.m to 1 .mu.m, for example), the feeler tip P cannot get close to the base B of the wall D upstanding perpendicularly to the surface S to be explored. The height H of the wall D will come into contact with one of the oblique faces of the pyramid of the tip, moving the apex of the latter away from the wall D by an amount proportional to the value of the angle defined above.
Note that finer tips with a more or less conical shape and a half-angle at the apex between 10.degree. and 20.degree., or less, have already been made by undercutting a silicon substrate, these tips also having a very small radius of curvature at their free end, typically in the range 5 nm to 20 nm.
However, as indicated hereinabove, these tips are not satisfactory because of their low hardness and premature wear. A description of a process of fabricating such tips is given in an article by T. Albrecht et al entitled "Microfabrication of cantilever styli for atomic force microscope" published in journal of Vacuum Science and Technology, Volume 8, page 3386, 1990.
An aim of the invention is to provide a process of fabricating a feeler member for a micromechanical probe having at least one tip of at least approximately conical shape and having a half-angle at the apex significantly smaller than that of tips made until now.